Side Effects of Flagyl (Metronidazole)

Flagyl is a type of antibiotic and an antiprotozoal medication that is used to treat bacterial infections in the body. The generic name for this antibiotic is metronidazole, and it also may be sold under the name protostar. Flagyl may be used to treat infections occurring in the skin, reproductive organs, and the digestive tract. It is often taken as a capsule, but it can also be given through an IV in a hospital or doctor's office.

Flagyl is sometimes used to treat Crohn's disease, which is one form of inflammatory bowel disease (IBD), especially when the disease is affecting the perianal area or only the colon (large intestine). It may also be used to treat pouchitis, which is a poorly-understood condition that occurs in people who have had surgery to remove their large intestine and create an internal pouch from the small intestine, which is called a j-pouch. Pouchitis tends to occur more often in people who had the surgery to treat ulcerative colitis than in the people who had the surgery to treat another condition in the colon, such as familial adenomatous polyposis (FAP). 

Flagyl is also used to treat an infection in the digestive tract with a bacteria called Clostridium difficile. C difficile infections are becoming increasingly prevalent and are especially dangerous when they occur in people who have IBD. For treating pouchitis and C difficile, Flagyl is sometimes taken for long periods of time, and perhaps along with another antibiotic, or alternating with another antibiotic.

Flagyl is also used in combination with other drugs to treat an infection with Helicobacter pylori. H pylori infects the stomach and can lead to stomach ulcers.

Check with your doctor if any of the following side effects continue or are bothersome:

• diarrhea
• dizziness or lightheadedness
• headache
• loss of appetite
• nausea or vomiting
• stomach pain or cramps

Less Frequent or Rare

Check with your doctor if any of the following side effects continue or are bothersome:

• change in taste sensation
• dryness of mouth
• unpleasant or sharp metallic taste

Less common:

• any vaginal irritation, discharge, or dryness not present before use of this medicine
• clumsiness or unsteadiness
• mood or other mental changes
• skin rash, hives, redness, or itching
• sore throat and fever
• stomach and back pain (severe)

For injection form:

• Pain, tenderness, redness, or swelling over vein in which the medicine is given

All antibiotics can reduce the effectiveness of oral contraceptives ("the pill"), so women using this as their birth control method should use a backup method until after finishing the course of antibiotics.

Flagyl can also sometimes cause urine to turn dark brown, but this is not dangerous and it goes away after the drug is stopped.

Other side effects not listed above may also occur in some patients. Check with a doctor about any other effects that occur. This information is meant only as a guideline—always consult a physician or pharmacist for complete information about prescription medications.